The present invention relates generally to marine electromagnetic surveying. More particularly, in one or more embodiments, this invention relates to an electromagnetic receiver assembly for marine electromagnetic surveying, the electromagnetic receiver assembly comprising an elongated housing and receiver electrodes mounted at separate points along the elongated housing, and methods of use thereof.
Marine electromagnetic (EM) surveying is a geophysical surveying technique that uses EM energy to identify possible hydrocarbon-bearing rock formations below the bottom of a body of water, such as a lake or ocean. In some instances, an EM source is towed through the water over an area of interest in the Earth's subsurface. The EM source can emit an energy field into the body of water that interacts with the rock formations below the water bottom. Without limitation, a number of EM receiver assemblies positioned on or near the water bottom detect changes in the energy field due to the interaction with the rock formation and generate response signals that can be used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
In conventional systems, the EM receiver assemblies may comprise a number of components, including a main body and EM sensors. The main body is typically a cube-like structure to which sensor electronics and an acoustic location system may be mounted. A ballast material (e.g., a concrete block) for deployment of the assembly onto the water bottom and a buoyant material for flotation may also be mounted to the main body. The EM sensors can include, for example, two, four, or more receiver electrodes mounted in the ends of long arms that can be attached to the main body. Drawbacks to these conventional EM receiver assemblies include the handling and logistics associated with deployment of the assemblies from a survey vessel. For example, due to space constraints associated with storing the EM receiver assemblies on the survey vessel, the EM survey systems may be limited to deployment of around 50 EM receiver assemblies. By way of further example, the arms as well as the ballast material are typically attached to the main body on the survey vessel when preparing for deployment, thus adding additional time and complexity to their deployment. With the ballast material attached, the bulky nature of the EM receiver assemblies adds to the difficulty in deploying them from the survey vessel. Recovery of the EM receiver assemblies after use may also be problematic as the equipment needed for recovery of the cube-like structure may add complexity to the EM survey system.
Accordingly, there is a need for improved receiver assemblies for marine EM surveying that can be more easily deployed and recovered than conventional receiver assemblies that have been used heretofore.